Method and apparatus for hot shaping magnesium alloy plates



y 1, 1941- w. M. VON TANNENBERG 7,

METHOD AND APPARATUS FOR HOT SHAPING MAGNESIUM ALLOY PLATES Filed Julv 5, 1938 2 Sheets-Sheet l y 1, 1941- w. M. VON TANNENBERG 7, 79

METHOD AND APPARATUS FOR HOT SHAPING MAGNESIUM ALLOY PLATES Filed July 5, 1938 2 Sheets-Sheet 2 Patented July 1, 1941 METHOD AND APPARATUS FOR HOT SHAP- ING MAGNESIUM ALLOY PLATES Walter Melnel von Tannenberg, Bremen, Germany, assignor to Leipziger Leichtmetall Werk Rackwitz, Bernhard Berghaus u. 00., Kom.- Ges., Rackwitz, near Leipzig, Germany Application July 5, 1938, Serial No. 217,612 In Germany November 20, 1937 6 Claims.

The shaping of plates of magnesium alloys has hitherto presented considerable difiiculty owing to the fact that the plates become so brittle as their strength increases that unexpected difficulties have hitherto arisen during their shaping.

The shaping of these magnesium alloy plates could hitherto be carried out without loss of strength and the formation of cracks only for a very narrow range of temperatures.

A number of methods are known in the working industry for the production of these temperatures. Thus, this temperature is produced particularly by gas burners, hot air appliances and electric resistances, but the accurate maintenance of a temperature interval, which should be for example 320il0 0., cannot be attained with this method at all or only with such complicated apparatus that its application in rough workshop service is impractical. Apart from the extremely expensive and inconvenient properties of this method from a work-shop point of view, the deformation is always accompanied by large waste because it is scarcely possible with such crude means to maintain a continually constant temperature and because the plate material undergoes undesirable deformation due to local stresses.

The invention relates to a method and apparatus for the hot shaping of plates especially of magnesium alloys, for aircraft construction, using pressure and heat by means of tool members relatively movable on the two sides of the plate to be shaped and ele tric currents, especially eddy currents.

The characteristic feature of this method is, that the current direction in the two leads is so chosen that the electric field. produced between the two leads is built up by the two leads and at the same time the lines of force which form around the leads have the same direction between the two leads and amplify the field.

Other features of the invention relate to the special development of the method especially as regards the mode of heating, the mode of feeding the current and the construction of the machinery.

The above mentioned objection is overcome by the invention chiefly in that the plates are heated quickly to the shaping temperature without heat conduction and without heat radiation, continually constant temperatures being attained without undesirable deformations taking place.

If an electrically conductive material is introduced into an electric eddy current field, its molecular movement increases proportionately to the field current intensity and consequently within the material a constant temperature is produced which is dependent, apart from the heat leakage, solely upon the intensity of the electric field. By a suitable selection of the field intensity the molecular movement can be so governed that brittle magnesium alloys can be perfectly shaped.

The drawings show the necessary devices for applying the method as follows:

Fig is an end view and shows schematically the conductors, between which an eddy field is formed;

Fig. 2 is a schematic perspective view of Fig. 1 with a wiring diagram added;

Fig. 3 is a diagrammatic perspective view showing the two conductors with the plate inserted between;

Fig. 4 is a ground plan of the device;

Fig. 5 is a cross sectional view through the clamping and bending jaws of one form of machine, in which the die process is used.

In the invention, the deformation of the plates takes place by means of high frequency alternating currents of great strength of current and low tension. A current is passed from an alternating current circuit to a transformer which is capable of transmitting great current strength by low tension.

The magnesium plates ii are not used as current carriers. Independent of the contact, it is possible through this invention to create within the plate I! which is to be worked, constant temperatures at which the work can be done without difiiculties.

For creating said temperatures, conductors I5, it are used which, while preventing current losses, connect the two poles of the two transformers by means of a loop of the smallest possible kind Through regulating the frequency, it is possible also to determine in advance the amount of current flowing through the conductors l 5 and IS. The correct measuring of the currents flowing through this conduit, however, forms around this conduit an electrical field of predetermined strength, built up in the same rhythm as that of the alternating field. The two conductors l5, I6 (Fig. 3) are therefore each contacted with a cur rent circuit H, I la and separate transformers I 0 and Ilia.

As will be seen from Figs. 1 and 2, the strengthening of the field is caused by the fact that the direction of current fiow is, as is shown by the arrows, in opposite directions in the conductors IS, IS. This causes the power lines K of the eddy field to run in the same direction at their contact periphery as shown by the arrows.

The flow of the electric current, together with the fall of the tension on the machine, is so subdivided that local burning cannot occur and the density of the current at each point of the material to be defprmed remains between 7 to 14 amperes.

According to the device shown in Fig. 3 the magnesium alloy plates are no longer used as current leads on the one hand or as heat conductors on the other hand. Independently of the contact of hot or very hot objects it is posslble, by the application of the invention, to produce in the plates to be shaped a constant temperature at which the working can be carried out without difficulty.

According to the invention high frequency machines may be used whose periodicity is regulatable and which include a transformer which converts this high frequency current into a current having a low voltage with extremely high current intensity. This apparatus also comprises a conductor which connects the two poles of the transformer in the smallest possible loo avoiding losses. By regulating the fre uency and the volta e respectively. it is also possible to predetermine the ouantities of electric current transported by the conductor However. the correct quantity of current passing through the conductor forms around this conductor an electric field of a certain field strength built up and decreasing in the same rhythm as the alternating field.

If an electrically conductive body is placed in this electric field. eddy currents are produced in this body at right angles to the field. These eddy currents when being checked. cause increases in temperature in the metallic body. By the increasing and decreasing electric field in the plate, the plate will be heated to the temperature necessary for shaping before it comes into contact at all with the tools under current.

It is possible, by employing different conductors, to form such an electric field in larger or small areas and consequentlv to carry out deformations of any desired superficial extent in plates of such magnesium alloys which are plastic only under the influences of certain temperatures.

To enable plates of ma nesium alloys to b bevel edged or curved, they must be uniformly heated to av d fracture during the bending of the plates. The heatin of magnesium alloy plates is effected by electric eddv f elds between the bendin rails whose cross sectional area is the same at every point of the wo k ng width. Thus, the current passed through the rails flows without any local chan ed resis ance. The bending rails according to the invention. are nowhere interrupted by screw holes and are insulated from he jaws of the machine by insulating strips (Fi 5). Thus, the current flowing through the rails does not leak into the jaws.

In order that the current flowing through the dies may not escape into the plates to be deformed and which are gripped between them, the dies are made out of so many single parts that a lower resistance is created between the beginning and end of each die part of the deformation tool than the transmission resistance from the plate in its cold condition and the dies.

When dimensioning the subdivided rails, it is also necessary to take into consideration that they must be adapted to the special constructional condition of the transformers coming in question for heating the rails, so that no separate feed rails are required and the circuit is very small.

Figs, 4 and 5 show one form of construction of the machine.

The plate I! is clamped in known manner between the upper jaw l8 and the lower jaw 19. It is held in position by the rails 20 and 2 I through which the electric current flows.

As soon as the plate I! has attained the temperature necessary for bending, it is bent over at an angle by raising the bending jaw 22. The bending rails 20, 2i and 23 are surrounded by insulating layers 20a, 2la and 23a respectively. In the jaws l8, l9 and 22 guide grooves 24 are provided in which the rails of V-shaped cross-section are clamped by clamping bars 25, 26, 21.

The clamping bars are pulled tightly against the jaws by bolts 28, 29 and 30. In Fig. 4 the subdivision of the rails 20, 2| and 23 into sections H is shown. One each of these sections feed and return bars or extensions b are fixed and carry the transformers.

One or two bending rails may be currentless, because they are preheated by eddy currents from the remaining current conducting rails.

To increase the economy, the jaws or the frame of the machine, may be made of nonmagnetic material, whereas the bending rails themselves are made of magnetic material.

The bending rails are attached to the jaws by separate shaped clamping bars which allow the expansion of the rails due to the heating.

The clamping bars fixed in the jaws with bolts and insulated against the rails, allow a slight adjustment so that a movement can take place in a direction parallel to the expanding rails.

To prevent greater heating at the points at which the current flows into the rails than at the current outflow, strips of sheet metal are soldered in and serve for conducting off the excessive heat.

I claim:

1. Apparatus for the hot deformation of plates especially of magnesium alloys for aircraft construction by means of pressure and heat and electric currents, especially eddy currents, comprising in combination a pair of electric circuits, relatively shiftable deforming tools, current conductors arranged in said tools, means connecting said conductors to separate circuits with the flow of current in said conductors being in opposite directions so that the electric fields forming about these conductors mutually amplify each other independently of the shape of the tools and produce molecular heating in the magnesium alloy plates to be deformed, said deforming tools being movable in different directions to simultaneously deform the plate in width, length and thickness owing to the molecular heating produced in the plate in the electric eddy current field.

2. A machine for edge bevelling and bending plates in arcuate shape, comprising in combination, relatively movable jaws, a circuit for each of said jaws, and rails carried by said jaws and each connected up to its circuit, the cross-sectional area of said rails being the same at every point on the entire working width of the machine, said rails being separated from the jaws by insulating layers, said bending rails being made of magnetic material and the jaws of nonmagnetic material.

3. A machine for edge bevelling and bending plates in arcuate shape, comprising in combination, relatively movable jaws, a circuit for each of said jaws, and rails carried by said jaws and each connected up to its circuit, the cross-sectional area of said rails beingthe same at every pointion the entire working width of the machine, said rails being separated from the jaws by insulating layers, separate clamping bars pressing said rails against said jaws, said bars forming guides allowing a rectilineal expansion of said rails under heat.

4. A process for the hot deformation of plates, particularly plates of magnesium alloys for aircraft construction by pressure and heat by means of dies relatively movable on the two sides of the plates to be deformed and of electric currents, comprising passing a separate current through each die member, creating an electric eddy field between said dies to heat said plate, said plate being formed in such manner that the direction of current is alternating and the force lines between the two conductors run in the same direction, and deforming said plate when the same is heated.

5. Method in accordance with claim 4, in which the length of the deformation tool conducting the current is sub-divided into such short parts that the necessary tension fall in these tools remains so low that the transmission resistance between the tool and said plate prevents entrance of the current into the plate.

6. Method in accordance with claim 4 in which the fiow of the electric current is so subdivided, in accordance with the cross section of the work piece and the tension fall so produced that local burning out cannot occur in the plate and the density of the current remains between 7-14 amperes at each point of the material to be deformed.

WALTER MEINEL VON TANNENBERG. 

